I.C. ENGINES
LECTURE NO: 10
(14 Apr 2014)
Fuel Spray Formation
• Spray Formation
Boundary
15 mm
15 mm
Core
Boundary
40 mm
75 mm
Fuel Spray Formation
• Fuel issues from the jet in a liquid stream
• The surface of the liquid come in contact with
air and the friction between the two results in
the formation of ligaments or threads, that
break into small particles and form an
envelope surrounding the core of the spray
• Core consist of highest velocity particles
Fuel Spray Formation
• Dispersion of the droplets in any one cross
section of the spray becomes more even:
• As the distance is increased from the orifice to
cross section
• As the air density is increased
• As the oil viscosity is decreased
• As the injection is increased
Fuel Spray Formation
• Measurement of the drop size indicate:
• Greatest number if droplets are less then 5
microns in diameter
• Increased the injection pressure decreased the
mean droplets size
• Increase the air density decreased the mean
droplet size
• Increase the oil viscosity increase the mean
droplet size
• Increase the orifice size increase the size of the
droplet
Fuel Spray Characteristics
• Degree of Atomization
• Penetration
• Dispersion
Fuel Spray Characteristics
• Diesel engine requires hard sprays because soft
sprays do not have adequate penetration in the
dense air
• Spray must be direct to various parts of the
combustion chamber by multiple orifices of the
nozzle or by using more than one nozzle in open
chambers in the absence of strong air motion
• Inlet inducted swirl is not necessary with divided
chambers. These chambers can give satisfactory
performances with single nozzle
• Spray duration at full load should not exceed 30˚
crank angle
Degree of Atomization
• Fuel velocity is the most important factor
affecting the degree of atomization
• 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ∞ 𝑝𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛 − 𝑝𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟
• Therefore increase the injection pressure reduces
the mean diameter of the particle as well as
variation in size
• Nukiyama and Tansawa develop an empirical
equation for computing the average drop
diameter which has the same surface –volume
ratio as that obtained by the entire spray
• 𝑑=
585 σ
+
𝑤 ρ
597 [
μ 0.45 1000𝑄𝑙 1.5
]
[
]
σρ
𝑄𝑎
Degree of Atomization
• 𝑑=
585 σ
+
𝑤 ρ
597 [
μ 0.45 1000𝑄𝑙 1.5
]
[
]
σρ
𝑄𝑎
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d = average drop diameter in microns (10-4 cm)
ω = relative velocity between air and liquid stream (m/s)
ρ = liquid density ( 0.7 to 1.2) ( g/cm3 )
σ = liquid surface tension ( 0.003 to 0.5) ( poise)
•
𝑄𝑙
𝑄𝑎
= 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑡𝑜 𝑎𝑖𝑟 𝑎𝑡 𝑣𝑒𝑛𝑎 𝑐𝑜𝑛𝑡𝑟𝑎𝑐𝑡𝑎
• This value is very small
• Therefore
σ
• 𝑑=
because surface tension is very important
ω ρ
585
Numerical Example
• Determine the average drop diameter for the
31.5 kgf/cm2 injection pressure. Values of fuel
density and surface tension may be taken as
0.86 g/cc and 28 dynes/cm respectively
• Formula
• ω = 𝐶𝑣
∆𝑝
2𝑔
ρ
Numerical Example
• Formula
• ω = 𝐶𝑣
∆𝑝
2𝑔
ρ
• = 0.94 2 ∗
31.5 ∗104
9.81
0.86∗ 103
• = 80 m/s
σ
585 28
• 𝑑=
=
80 0.86
ω ρ
• = 42 𝑚𝑖𝑐𝑟𝑜𝑛𝑠 = 42 ∗ 10−4 𝑐𝑚
585
Penetration
• Jet Velocity
• An increase in injection pressure increase jet
velocity
• Spray tip penetration increases with jet
velocity
• Air Density
• An increase in combustion chamber air
density decreases the penetration
Penetration
• Orifice Diameter
• An increase in orifice diameter increase the
penetration of the spray tip.
• Orifice length to diameter ratio between 4:1
and 6:1 results in maximum penetration.
• The minimum penetration is reached with
ratio 1:1 and 3:1
Penetration
• Orifice Diameter
• As per schwitzer
• 𝑆 = 𝑓1 (𝑡 ∆𝑝
•
𝑆
𝑑
•
•
•
•
•
•
𝑆 1 + 𝑑𝑛 = 𝑓3 (𝑡, 𝑑𝑎 )
Where
S = Penetration time
𝝙p = Pressure difference across orifice
d = Orifice diameter
da = air density in atm
= 𝑓2
𝑡
𝑑
Numerical Example
• Penetration of 20 cm in 15.7 millisec is
obtained with 140 kgf/cm2 injection pressure.
Values of fuel density and surface tension may
be taken as 0.86 g/cc and 28 dynes/cm
respectively
• Formula
• ω = 𝐶𝑣
∆𝑝
2𝑔
ρ
KEY TERMS
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Electronic ignition system (EIS)
Electronic spark timing (EST)
Flyback voltage
Hall-effect switch
High energy ignition (HEI)
Igniter
Ignition coil
Ignition control (IC)
Ignition control module (ICM)
Ignition timing
Inductive reactance
Initial timing
Ion-sensing ignition
Iridium spark plugs
Knock sensor (KS)
Magnetic pulse generator
Magnetic sensor
Married coil
Mutual induction
Optical sensors
Paired cylinder
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Pickup coil (pulse generator)
Ping
Platinum spark plugs
Polarity
Primary ignition circuit
Saturation
Schmitt trigger
Secondary ignition circuit
Self-induction
Spark knock
Spark output (SPOUT)
Switching
Tapped transformer
Transistor
Trigger
True transformer
Turns ratio
Up-integrated ignition
Waste-spark ignition
Function
• An ignition system is a system for igniting a fuelair mixture at the right instant.
• It is best known in the field of internal
combustion engines but also has other
applications, e.g. in oil-fired and gas-fired boilers.
• Hot spark across spark plug gap
• Distributes high voltage to each plug in correct
sequence
• Time the spark so it arrives as piston nearing TDC
• Adjusts spark timing with load & speed
History
• The earliest internal combustion engines used
a flame, or a heated tube, for ignition
• These were later replaced by systems using an
electric spark. The instant of sparking is
decided by the ignition system.
FUNDAMENTAL ELECTRICAL PRINCIPLES
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Electricity is lazy
Electricity wants to go to ground
electron theory (-) to (+)
conventional theory (+) to (-)
Conductors
Insulators
ELECTRICAL UNITS OF MEASUREMENT
• Volts---- Push
• Current ---Quantity
• Resistance ----Resistance to flow
V
A

OHM’S LAW
• E=IxR
• E/I=R
• E/R=I
E
I
R
MAGNETISM
• Alike charges repel (-) (-)
• Dissimilar charges attract (-) (+)
MAGNETS & ELECTRICITY
• Magnets can be used to for electricity
• Electricity can be used to form magnets
• Electricity when applied to magnets make
stronger magnets
IGNITION COILS
• Coils of wire wrapped around an iron core
• Step up transformer
SPARK PLUGS
• Spark plugs
contain an air gap
for electricity to
create a spark
and make a seal
HEAT RANGES
The difference between a "hot" and a "cold" spark plug is
in the shape of the ceramic tip.
• The manufacturers will select the right-temperature plug
for each engine.
• Some engines with high-performance naturally generate
more heat, so they need colder plugs.
• If the spark plug gets too hot, it could ignite the fuel before
the spark fires
• It is important to stick with the right type of plug
• Engine that burn oil may need hot plugs
MEASURING SPARK PLUG TEMPERATURE
TYPES OF ELECTRODES
• Center electrode
• Side electrode
SWITCHING DEVICES
• Breaker points
• Electronic
BREAKER POINTS
• Ran by cam shaft
ELECTRONIC SWITCHING DEVICES
• NO breaker points to burn or wear out
Basic Ignition System Operation
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Charge builds up in coil (12 volts in)
Creates a magnetic field (windings of wire)
Voltage is stepped up (secondary windings)
Switch open (magnetic field collapses)
High voltage discharged (to plug)
IGNITION SYSTEM
•Provides a method of turning a spark ignition engine on & off.
•Operates on various supply voltages (Battery & Alternator)
•Produces high voltage arcs at the spark plug electrode.
•Distributes spark to each plug in correct sequence.
•Times the spark so that it occurs as the piston nears the TDC on the
compression stroke.
•Varies the ignition timing as engine speed, load and other conditions
change.
IGNITION PARTS
BATTERY provides power for system.
IGNITION SWITCH allows driver to turn ignition on and off.
IGNITION COIL changes battery voltage to 30,000V during
normal operation and has a potential to produce up to 60,000V.
SWITCHING DEVICE mechanical or electronic switch that operates
Ignition coil(Pick-up coil, Crank sensor, Cam sensor).
SPARK PLUG uses high voltage from ignition coil to produce an arc
in the combustion chamber.
IGNITION SYSTEM WIRES connect components.
IGNITION CIRCUITS
PRIMARY CIRCUIT
•Includes all the components
working on low voltage
(Battery, Alternator).
SECONDARY CIRCUIT
•Consists of wires and points
between coil out-put and the
spark plug ground.
IGNITION COIL
Primary Windings are made up of several
hundred turns of heavy wire wrapped around
or near the secondary windings.
Secondary Windings consist of several thousand
turns of very fine wire, located inside or near
the secondary windings.
DISTRIBUTOR
•Actuates the on/off cycle of current flow through the ignition coil primary
windings.
•It distributes the coils high voltage to the plugs wires.
DISTRIBUTOR
•It causes the spark to occur at each plug earlier in the compression stroke as engine speed
increases, and vice versa.
•Changes spark timing.
•Some distributor shafts operate the oil pump.
POINT IGNITION SYSTEM
PARTS Distributor Cam, Breaker Points, and Condenser.
POINT IGNITION SYSTEM
Points are wired in Primary Circuit – When the points are closed, a
magnetic field builds in the coil. When the points open, the field
collapses and voltage is sent to one of the spark plug.
DISTRIBUTOR CAP
•Insulated plastic cap
•Transfers voltage from coil (wire) to Rotor.
DISTRIBUTOR ROTOR
•Transfers voltage from the distributor cap
center terminal(coil) to distributor cap
outer terminals(spark plugs).
•Provides spark in the correct Firing Order.
•Sometimes the firing order can be found
on the intake manifold.
IGNITION TIMING
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BTDC
ATDC
Engine RPM
Engine Load
Firing Order
Retard
Advance
FIRING ORDER
FIRING ORDER
• 1,3,4,2
• 1,2,5,4,3,2
• 1,5,6,3,4,2,7,8
FIRING ORDER
CONDENSER
• High voltage is developed in the secondary
ignition coil.
• Similarly “Back EMF” is produced in the
primary coil (could cause a spark on the
primary end) due to sudden collapse of
magnetic field.
• The condenser prevents this by
slowing down the rate of collapse.
SPARK PLUGS
• Used in SI engines
• Function
– Starts the combustion process when the piston is at the
TDC.
– Electricity converted in to spark by forcing electricity to arc
across a gap, just like a bolt of lightning.
• Salient Features
– Voltage at the spark plug can be anywhere from 40,000 to
100,000 volts.
– Spark plugs also transfer heat away from the combustion
chamber.
SPARK PLUG PLACEMENT
PARTS OF A SPARK PLUG
• Connector (terminal) –
connects sparkplug to the
ignition system.
• Ceramic Insulator – Provides
mechanical support to the
central electrode.
• Resistance - Copper core
which connects from the
connector and surrounded by
insulation.
• Spline (ribs) – Improves
insulation by providing more
resistance to electricity.
• Gasket (metal) – arrests
leakage from the combustion
chamber.
www.howstuffworks.com
PARTS OF A SPARK PLUG CONTD..
www.infovisual.info
• Spark plug body – Metal case
serves to remove heat from the
insulator and transfer to
cylinder head. Also acts as a
ground for the spark passing
from the central electrode to
the ground electrode.
• Central electrode – connected
to the terminal through a
resistance in series. Usually
made of a copper alloy.
• Ground electrode - Made of
nickel steel and welded to the
spark plug body.
• Spark plug gap – Gap between
the central electrode and
ground electrode
TYPES OF SPARK PLUGS
• Made of ceramic inserts
• Has smaller contact area
with the metal part of plug
• Runs hotter and burns away
carbon deposits
• Used in most standard
engines
• Designed with more contact
area and less thermal
insulation
• They run cooler
• Used in high compression
ratio – high power engines
SPARK PLUG GAP
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Typically designed to have the spark gap adjusted by bending
the ground electrode slightly to bring it either closer or
further from the central electrode.
Spark plugs in automobiles generally have a gap between
0.045"-0.070" (1.2-1.8mm).
Spark plug gauge
– A disc with a sloping edge, or with round wires of precise
diameters, which is used to measure the gap
– a collection of keys of various thicknesses which match
the desired gaps and the gap is adjusted until the key fits
snugly.
The main issues with spark plug gaps are:
– narrow-gap risk: spark might be too weak/small to ignite
fuel;
– narrow-gap benefit: plug always fires on each cycle;
– wide-gap risk: plug might not fire, or miss at high speeds;
– wide-gap benefit: spark is strong for a clean burn.
Disc gauge
SPARK PLUGS TELL A STORY
Normal
Over Heating
Worn
Carbon
Lead Erossion
Fuel/Additive
Deposits
Insulator Breakage
Lead Fouled
Lead Fouled
Minor Melting
Oil
ESTIMATING ENGINE CONDITION
• Spark plug's insulator color provides valuable information
about the engine's overall operating condition.
• Normal: Grey to Light Golden-Brown Color
– This condition is ideal, the spark plug and engine
air/fuel mixture are operating properly.
• Dry Fouling: Black Soot Buildup
– Air/fuel mixture is too rich, the carburetor settings
are incorrect, or the flame arrestor is dirty or has
mounting problems.
– Spark plug heat range is too cold for the
operating
conditions.
– Ignition system problems causing a weak or
intermittent spark.
ESTIMATING ENGINE CONDITION CONTD..
• Wet Fouling: Shiny, Wet, Black Appearance
– Excessive use of the choke (gas fouled)
– Prolonged low rpm operation (gas or oil fouled)
– Fuel to oil ratio is too rich (oil fouled)
• Excess Deposits: Bumpy, Chalky Buildup
– Poor fuel quality
– Oil leakage into combustion chamber
– Improper oil used for premix/injected
• Detonation: silver or black specs, melting or
breakage at the firing tip
– Caused by improper timing
– Lean air/fuel mixture can aggravate this condition
ESTIMATING ENGINE CONDITION CONTD..
• Overheated: White, Blistered, Melted Electrode
– Lean air/fuel mixture
– Spark plug heat range is too hot for engine
operating condition
– Plug is not properly gapped and/or tightened onto head
– Overly advanced timing
• Breakage: Sooty appearance, missing or damage
components of the spark plug
– Caused by thermal expansion / contraction of the
insulator due to thermal shock
– Sudden decreases in temperature can most commonly be coincided
with entering a large pool of water while the engine is hot, or a broken
water jacket for liquid-cooled engines.
SPARK PLUG WIRES
• Very high resistance
wire 1000 ohms per
inch
• Mostly
insulation
material
• Small
conductor
material
• Must follow firing
order
IGNITION TIMING
How early or late the spark plug fires in relation to the
position of the engine piston.
Ignition timing must change with the changes in engine
speed, load, and temperature.
IGNITION TIMING
Timing Advance occurs when the plug fires sooner on
compression stroke (High engine speed)
Timing Retard occurs when plug fires later on compression stroke
(Lower engine speed)
BASE TIMING Timing without vacuum or computer control.
METHODS OF CONTROLLING TIMING
Distributor Centrifugal Advance
•Controlled by engine speed.
•Consists of two weights and two springs.
•At high speeds the weights fly out(held by the springs), rotating the
cam, hence advancing the timing.
METHODS OF CONTROLLING TIMING
Vacuum Advance
•Controlled by engine intake manifold vacuum and engine load.
•The vacuum diaphragm rotates the pickup coil against the direction
of distributor shaft rotation.
METHODS OF CONTROLLING TIMING
Electronic Advance Sensors input influences the ignition timing.
•Crank shaft Position Sensor (RPM)
•Cam Position Sensor (tells which
cylinder is on compression stroke)
•Manifold Absolute Pressure (MAP)
(engine vacuum and load)
METHODS OF CONTROLLING TIMING
Electronic Advance Sensors input influences the ignition timing.
•Intake Air Temperature Sensor
•Knock Sensor (Retards timing when pinging
or knocking is sensed)
•Throttle Position Sensor(TPS)
•Engine coolant Temperature
IGNITION SYSTEM
Distributor VS Distributor Less Ignition System
DISADVANTAGE OF THE MECHANICAL SYSTEM
• Breaker contact points require regular replacement
because
– points are subject to mechanical wear where they ride the
cam to open and shut
– oxidation and burning at the point contact surfaces from
the constant sparking.
• Spark voltage is also dependent on contact
effectiveness, and poor sparking can lead to lower
engine efficiency.
• Beyond average ignition current ~ 3A, service life
reduces, thus limiting the power of the spark and
ultimate engine speed.